a. Field of the Invention
The invention relates to the field of the rigging of plural-hull sailing craft, and particularly concerns an arrangement for removing rigging forces from the hulls, and for reducing the downthrust of the mast upon the hull-interconnecting structure.
B. Description of Prior Art
Of all the various types of plural-hull sailing craft only the trimaran possesses a conventional mounting base for forestays and headsail tacks, on the foredeck of the centrally-located main hull. In catamarans, proas, and other types of plural-hull sailing craft such as that described in my U.S. Pat. No. 3,933,110 and my copending application Ser. No. 825,441, the headsail tack is located in air over water, as is the lower end of the forestay. This presents a difficulty in securely attaching the forestay and headsail tacks to the sailing craft. In the past this was usually overcome by attaching twin cables to the bows of the leading hulls; the free ends of these cables were connected to each other and to the forestay, and the headsail tack was connected either to the junction of the cables or to the forestay in the vicinity of the junction -- an arrangement usually called a bridle. This arrangement was simple and light, and seemed to add very little weight to the craft, when compared to comparable craft without headsails. However, the small bridle weight is deceptive, for by reason of its geometry the stress it places on the bows is so high as to tend to break the hulls off at the forward hull-interconnections. This requires that the hulls be heavy enough to withstand these substantial forces. A typical bridle places an upward pull on each bow of half the vertical component of combined headsail-forestay pull on the bridle: it places a horizontal inward pull on each bow whose magnitude depends on the angle between the legs of the bridle. The greater that angle the greater the inward pull. There is a tendency to keep the headsail low, to reduce the heeling forces on the craft, which causes a greater angle, resulting in a magnification of the vertical force component on the bridle. It is not unusual for these horizontal components to have a magnitude of over twice the vertical components on the bows. Normally the hulls are narrow and deep, thereby having a higher strength in the vertical, yet must resist greater horizontal forces. These forces are exerted at distances over four feet, in light one-man daysailers, to over twelve feet in larger racing craft. Consequently the hulls must withstand breaking moments of from hundreds to thousands of foot-pounds. Since these hulls are normally not fabricated of materials having high ratios of strength-to-weight, the resulting increase in hull weight is often appreciable.
In certain cases a compression member is connected from bow to bow to resist the inward pull. Usually a long, slender, sometimes streamlined, aluminum tube, this compression member's actual ability to resist the inward hull-breaking force is far less than the compressive strength of the tube. The ratio of length of the tube to its radius of gyration is so high that the tube is actually operating in the regime of very long columns, whose drastically-reduced resistance to buckling loads is well known. In addition, wave slap, bouncing and vibration, and the pull of gravity, cause this member to deflect from a straight line, such that it then acts more like a spring than a column. Thus the tube cannot totally remove the extra hull weight, but is usually employed as a secondary means of resistance. In some cases this tube is only remedial, being applied after design, to craft which have exhibited a tendency to hull failure. Tubes large enough in diameter to act as compression members would increase the wind resistance of the sailing craft, would approach the size of the hull-interconnecting structure and hence be considered unsightly and cumbersome by some, so most manufacturers place the needed strength where it is not so apparent to the eye.
In many old-fashioned monohull sailing craft the presence of a bowsprit and/or a jibboom provided a mounting base for stays and tacks which would otherwise be located in midair over water. These spars were securely housed in the bows of the craft; most plural-hull sailing craft have not been able to adopt this approach for they lack bows in the proper location.
Accordingly, it is an object of this invention to remove the forces exerted by forestays and headsail tacks from the hulls of plural-hull sailing craft, and to eliminate the danger of hull breakage resulting from such forces. It is another object of this invention to reduce the weight of the hulls of plural-hull sailing craft.
In plural-hull sailing craft the mainsail sheet also exerts an upward pull; usually this is resisted chiefly by the rearmost hull-interconnecting cross-member. This pull is transmitted to the hulls and the foremost cross-member; the hulls being enabled to resist this force since the ends of the cross-members are usually rigidly attached to substantial side beams, which act in concert with the hulls; the side beams being usually fabricated of a material having a high ratio of strength-to-weight. However, the side beams are thus heavier than required by their primary task of supporting the deck trampoline and crew.
Accordingly, it is another object of this invention to reduce the forces exerted upon the side beams by the pull of the mainsail sheet, and thereby to reduce the weight of the side beams of plural-hull sailing craft.
The sheet force has a counterpart in the downward pull that the head of the mainsail exerts upon the upper region of the mast. In the same manner, the forestay, the headsail, the shrouds, the backstay (if present), and the weight of the sails and rigging, also exerts a downward pull of the upper region of the mast. The sum of these forces represents the compression loading on the mast, which together with the weight of the mast, exert a substantial downthrust force upon the mast step and that portion of hull-interconnecting structure which carries the mast step - usually the foremost cross-member, called the main beam. This downthrust is the greatest concentration of forces exerted anywhere in the craft. The downthrust is so severe, especially in high winds, that it is not unknown for fast, lightweight racing craft to suffer a bending failure of the main beam under the mast step. Often a compression member (called a dolphin-striker) is attached below the main beam under the mast seat to transfer much of the downthrust to lateral tension members connected also to the outer ends of the main beam. In short, the main beam is made into a truss.
Accordingly, it is another object of this invention to reduce the forces exerted upon the main beam by mast downthrust, and thereby reduce the weight of the main beam truss.
Certain plural-hull sailing craft, notably the proa or calie, reverse the direction of travel when tacking through the wind, in order to keep the shorter hull to windward. These must usually move rudders, stays, and sails to opposite ends of the craft to complete the maneuver.
Accordingly, it is another object of this invention to provide a base for the rigging of reversing plural-hull sailing craft, which base has reversibility.
In the plural-hull sailing craft of my U.S. Pat. No. 3,933,110 and my co-pending application Ser. No. 825,441, the configuration of the relationship of the hulls undergoes a mirror-image transformation when the craft tacks or gybes. Consequently the locations of the headsail tack and the forestay end and of the mainsail sheet trim point must also undergo a mirror-image transformation. Since the hulls are pivotally connected to the hull-interconnecting structures (called spyders) this causes extra difficulties in the use of bridles, in addition to the geometric constraints imposed by the requirements of mirror-image transformation.
Accordingly, it is another object of the invention to provide a mounting base for rigging of plural-hull sailing craft whose hulls and sails undergo a mirror-image transformation while tacking or gybing.
With a fixed location for the headsail tack the only control for the volume and velocity of air flowing through the space between an overlapping headsail and a mainsail (called the slot) is by changing the tension of the headsail sheet, to change slightly the amount of convergence of the cross-sectional airfoil shape of the two sails, or to move the headsail sheet trim point laterally to change the width of the slot exit, thus not appreciably affecting the shape of either airfoil but appreciably changing the amount of convergence of the two airfoils. If the trim point is moved inwards too much, in an attempt to increase airflow velocity in the slot, the convergence will be too great, causing the air to impinge too directly on the lee side of the mainsail, instead of flowing along the lee side of the mainsail. This will cause the mainsail to be backwinded; and it will bulge appreciably to windward, spoiling the slot's enhancement of sail efficiency. If the trim point is moved out too far the airfoil curves will not converge, or even may diverge; airflow velocity is reduced and sail efficiency declines.
Accordingly, it is another object of the invention to provide better control of the volume of air intercepted by the headsails of plural-hull sailing craft, thereby providing additional control of slot airflow velocity and sail efficiency.
In certain monohull racing boats the headsail tack attachment point may be moved fore-and-aft to obtain a better balance with the forces of lateral resistance in the water, to make steering of the craft easier and more sensitive. Sail trimming and fine tuning of the rig can also be affected, giving the operators of the craft another controllable variable. This feature is not available with bridles. While conceivably the junction of the bridle cable could be hauled back and forth slightly, this would tend to increase the forestay tension drastically, for a very small range of movement.
Accordingly, it is another object of the invention to provide longitudinally variable position of headsail tack attachment, for better sail trim, rig tuning, and balance for steering.
Similarly, it would also be desireable to move the position of the mainsail sheet trim point fore-and-aft, and to eliminate the boom. Since most plural-hull sailing craft have no physical base for attaching the main sheet trim point as far aft as directly under the mainsail clew, a boom must be used, with the sheet blocks attached centrally, in the vicinity of the aft beam. If mounting were provided below and after the main clew for the mainsail sheet trim point, the moveable positioning mechanism would serve to outhaul a mainsail without a boom, for shape control. If also the aftward extending mounting base for the mainsail sheet trim point could be controllably deflected laterally, the mainsail sheeting angle could also be varied.
Accordingly, it is another object of the invention to provide a mounting base for a boomless mainsail's sheet. It is another object to provide longitudinal adjustment for the mainsail sheet trim point. It is yet another object of the invention to provide mainsail sheeting angle control for boomless mainsails in plural-hull sailing craft.
It would be another desireable feature in a plural-hull sailing craft to make the mainsail sheet to be self-tending when the craft tacks, such that the mainsail sheet trim point is carried over to an equivalent position on the other tack.
Accordingly, it is another object of the invention to provide a self-tending capability for mainsail sheets in plural-hull sailing craft.